This interactive electronic report provides an overview of an innovative new instrument developed by researchers at the Consortium for Policy Research in Education (CPRE) to authentically measure teachers' formative assessment practices in mathematics. The Teacher Analysis of Student Knowledge, or TASK, instrument assesses mathematics teachers' knowledge of formative assessment and learning trajectories, important components of the instructional knowledge necessary to teach to the high expectations of the Common Core State Standards (CCSS). The electronic report has three main sections: the first gives an overview of the TASK instrument, the second provides a sample TASK in fractions, appropriate for teachers in grades 3-5, and the third provides an overview of the domains measured by the TASK instrument and reports the results of a major national field trial. These results were collected from five urban and urban fringe districts in five states in the northeast and southern United States. They provide a sense of current teacher capacity to meet the ambitious expectations of the Common Core State Standards in Mathematics.